Lighting systems based on LEDs have an advantage over traditional fluorescent lighting systems in that they can be controlled to vary both their color and brightness. Through an appropriate combination of these two parameters, subtle lighting effects such as sunrise, sunset, and mood lighting can be achieved. Because of this and other advantages, LED based lighting systems are rapidly replacing traditional fluorescent lighting systems in a number of environments, including transportation, military, commercial, and home environments.
A typical LED based lighting system employs at least three different colors of LEDs, for example, red (R), green (G), and blue (B), arranged in a repeating array. The colors can be combined in different proportions to produce thousands of resultant colors (e.g., white) when viewed by the human eye. FIG. 1 shows a general layout of a portion of a basic LED based lighting system 100. The system 100 has a microprocessor 102, a power supply 104, and one or more arrays of LEDs 106, 108. Each array 106, 108 includes a number of red, green, and blue LEDs, one of which is shown at 110. Within each array, LEDs that emit the same color have their cathode and anodes electrically tied together to form a channel. Each channel is controlled by the microprocessor 102 independently of the other channels, both within a given array and also from array to array in some cases.
Although only one microprocessor 102 is shown, most applications will have multiple microprocessors 102 controlling multiple sets of LED arrays 106, 108. It is also possible to have one microprocessor controlling a single LED array. In a typical installation, one microprocessor controls one light fixture, which can include several LED arrays. Additionally, although the arrays 106, 108 show an equal number of red, green, and blue LEDs, in some applications, there may be more or fewer of one color than others. Moreover, some arrays may have a higher or lower total number of LEDs than others. It is also possible to have an array with only one LED for each of the constituent colors.
In operation, a user selects the resultant light output (e.g., white) for the desired LED arrays 106, 108 via a global control unit (not expressly shown). The global control unit then relays the desired resultant color to the appropriate microprocessors 102. Each microprocessor 102 controls the relative intensity level of the channels within its set of arrays to achieve the desired resultant color. The intensity level of a channel is controlled using pulse-width modulation (PWM) of a fixed current source (e.g., 20 mA, 40 mA, etc.) provided by the power supply 104. The power supply 104 is connected to the LED arrays 106, 108 via a plurality of switches (not expressly shown) in either a common anode or a common cathode configuration. Each microprocessor 102 then modulates the current for its LED channels by turning on and off the appropriate switches, which may be any suitable switching device (e.g., a field effect transistor (FET)).
Within each microprocessor 102, a PWM algorithm 114 adjusts the PWM for each channel to achieve the desired resultant color. PWM allows linear control of the intensity level of the LEDs; that is, the intensity level of an LED is directly proportional to the width of the pulse. The algorithm 114 uses pre-stored calibration data to determine the PWM that should be applied to each channel to achieve the resultant color. By varying the PWM of the different channels, it is possible to generate thousands of different resultant colors.
Successful execution of the above LED based lighting system 100 requires that the individual LEDs 110 within a channel of a given array be closely matched in intensity (brightness) and wavelength (color.) Ideally, every LED of a particular color would have the same intensity and wavelength. In reality, the LED manufacturing process is not perfect, and manufacturers have to bin (sort) the LEDs by intensity and wavelength. For an LED of a given color, it is common to have six or more different bins. Thus, an array with three different colors could theoretically require up to 216 different bins to cover all the possible combinations.
The logistics of managing so many bins is both complex and expensive. A more economic solution is to closely match the intensity and wavelength of LEDs of a given color on a single array, but permit variation from array to array. A calibration process can then be used to normalize the resultant output on an array by array basis so that multiple arrays produce substantially the same color when governed by a global command. The calibration process generates the maximum possible intensity at a specific color target. The color target is identified using the CIE (Commission International de l'Eclairage) chromaticity coordinates u′ and v′. The coordinates for “High White,” for example, are u′=0.2221 and v′=0.5024. Any combination of red, blue, and green that results in these coordinates will produce a High White color. The PWM for each channel that generates the maximum intensity level at the target color is then stored in the microprocessor.
Unfortunately, the intensity level of an LED changes with temperature, which can cause a drift in the LED's color. And while the human eye is relatively insensitive to variations in the intensity (brightness) of a given color, it is very sensitive to variations in the color (wavelength) itself A shift of 3-4 nanometers in the wavelength of a LED creates a color shift that is noticeable to the human eye. Since the resultant color generated by the lighting system is composed of three or more colors, a minor shift in output of any one of the component colors may lead to visible color shifts. The charts in FIGS. 2A-2B show examples of bow the u′ and v′ parameters can change with temperature. As the temperature of the LED rises, its output drops, which changes the u′ and v′ parameters. This intensity drop varies for each LED color, since the different colors are based on different LED technologies.
In typical installations, the LED arrays for the same microprocessor and/or different microprocessors are placed in different locations and may be exposed to different thermal environments. For example, in airplane cabin lighting systems, one array or set of arrays may be installed under an air conditioning vent while a second array or set of arrays is installed in a warmer spot. Since the LEDs at the different locations now operate at different temperatures relative to each other, the resultant color produced at those locations will be different relative to each other. If the difference in temperature is large enough, there will be a significant variation in output color between the locations.
Accordingly, what is needed is a way to compensate for thermal differences in LED based lighting systems. More specifically, what is needed is a way to adjust the PWM of the LEDs to compensate for differences in the temperature of the operating environment.